A review of the geologic history of the Himalayan-Tibetan orogen suggests that at least 1400 km of north-south shortening has been absorbed by the orogen since the onset of the Indo-Asian collision at about 70 Ma. Significant crustal shortening, which leads to eventual construction of the Cenozoic Tibetan plateau, began more or less synchronously in the Eocene (50–40 Ma) in the Tethyan Himalaya in the south, and in the Kunlun Shan and the Qilian Shan some 1000–1400 km in the north. The Paleozoic and Mesozoic tectonic histories in the Himalayan-Tibetan orogen exerted a strong control over the Cenozoic strain history and strain distribution. The presence of widespread Triassic flysch complex in the Songpan-Ganzi-Hoh Xil and the Qiangtang terranes can be spatially correlated with Cenozoic volcanism and thrusting in central Tibet. The marked difference in seismic properties of the crust and the upper mantle between southern and central Tibet is a manifestation of both Mesozoic and Cenozoic tectonics. The form...

Geologic Evolution of the Himalayan-Tibetan Orogen

http://igppweb.ucsd.edu/~fialko/rheo/Carter_Tectonophysics_1987.pdf

Flow properties of continental lithosphere

SUMMARY

We present geological and morphological observations at different scales to constrain rates of faulting and the distribution of deformation in the seismically active Aegean region. We focus first on the 130 km long Corinth Rift, an asymmetric graben where a flight of terraces of marine origin are uplifted. We show that the edges of the terraces lie in the footwall of the normal fault bounding the Corinth Rift and correspond to sea-level highstands of laic Pleistocene age. Using a detailed analysis of aerial and SPOT imagery supported by field observations, we have mapped 10 terrace platforms and strandlines ranging in elevation from 10 to 400 m over distances of 2 to 20 km from the fault. The elevation of the terraces' inner edges was estimated at 172 sites with an error of ±5m. This data set contains a precise description of the uplift and flexure of 10 different palaeohorizontal lines with respect to the present sea level. To date the deformation, we correlate the Corinth terraces with late Pleistocene oxygen-isotope stages of high sea-level stands and with global sea-level fluctuations. Using a thick elastic plate model consistent with our current understanding of the earthquake cycle and a boundary-element technique we reproduce the geometry of the shorelines to constrain both mechanical parameters and the slip on the fault. We show that the seismogenic layer behaves over the long term as if its elastic modulus were reduced by a factor of about 1000. All the terraces are fitted for fault slip increasing in proportion to terrace age, and the component of regional uplift is found to be less than 0.3 mm yr−1. The best fits give a slip rate of 11±3 mm yr−1 on the main rift-bounding fault over the last 350 kyr. Other geological and morphologic information allows us to estimate the total age of the main fault (∼1 Ma) and to examine the mechanical evolution of the Corinth Rift. The minimum observed sediment thickness in the Gulf places an extreme check on the results of the modelling and a lower bound on slip rate of 6–7 mm yr−1 (40 per cent less than estimated with modelling). Even this slip rate is nearly 10 times higher than for comparable features in most of the Aegean and elsewhere in the world.



At a larger scale, the spacing and asymmetry of the rift systems in the Aegean suggest strain localization in the upper mantle, with slow extension starting 15 Myr ago or earlier. The more recent (1 Myr), rapid phase of rifting in Corinth partly reactivated this earlier phase of extension. The younger faulting in Corinth appears to result from its present location in the inhomogeneous stress field (process zone) of the south-westward propagating tip of the southern branch of the North Anatolian Fault. We extend these relations to propose a mechanical model for the Late Cenozoic evolution of the Aegean. As the Arabia/Europe collision progressed in eastern Turkey it caused Anatolia to move to the west and the North Anatolian Fault to propagate into the Aegean, where the early slow extension started to be modified about 5 Ma ago. The process of propagation dramatically increased the activity of some but not all of the earlier rifts. The model we present is compatible with tectonic observations, as well as with the seismicity, the palaeomagnetic rotations and the displacement field now observed with GPS and SLR.

Quaternary evolution of the Corinth Rift and its implications for the Late Cenozoic evolution of the Aegean

A conceptual model is proposed for the generation of low-angle normal faults in Metamorphic Core Complexes. The model is based on three assumptions: (1) the isostatic response to normal fault motion is of regional extent; (2) when a fault segment is significantly rotated from the optimum angle of slip, relative to the crustal stress field, it is replaced by a new planar fault oriented in the optimum direction; and (3) the fault cuts the entire upper crust and fault motion always nucleates in the same region at the base of the upper crust. The stress field is considered to be uniform through the crust and the regional isostatic response of the crust to loads is computed using the thin plate flexure approximations. Active low-angle normal faults are difficult to reconcile with rock mechanics theories, earthquake focal mechanism studies, and geochronologic results indicating rapid cooling of core complex rocks. The model does not require active fault slip on low-angle faults. The flexural response to normal faulting is shown to be significantly affected by anelastic behavior of the crust and by loading due to sedimentation. The anelastic response to large bending stresses results in significant reduction in the effective elastic rigidity of the upper crust. This can explain the observed short wavelength of topographic response to normal fault loads. The model results in: (1) a nearly flat-lying abandoned normal fault (or “detachment”) below slices of upper plate rocks and sedimentary infill; (2) a strong contrast in metamorphic grade across the abandoned “detachment”; (3) rapid movement of lower plate rocks from midcrustal depths to shallower depths. These results are qualitatively in agreement with geologic observations for core complexes.

Flexural rotation of normal faults

We review recent results concerning the rheology of the lithosphere with special attention to the following topics: 1) the flexure of the oceanic lithosphere, 2) deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins, 3) the rheological stratification of the continents, 4) strain localization and shear zone development, and 5) strain-induced crystallographic preferred orientations and anisotropies in body-wave velocities. We conclude with a section citing the 1983–1986 rock mechanics literature by category.

Rheology of the lithosphere: Selected topics

To investigate the behavior of the lithosphere undergoing extension, we use a simple rheological model broadly consistent with experimental data on rock creep and with the nature of the brittle/ductile transition. A plastic surface layer overlies a substrate that deforms by power law creep with a stress exponent n = 3 and an effective viscosity that decreases with depth. In extension this model shows a strong necking instability, provided that the thermal gradient is sufficiently large; otherwise, stable uniform extension is indicated. The predicted structures display uniformly spaced necks or regions of enhanced extension (basins) alternating with regions of reduced extension (ranges). If the depth to the brittle/ductile transition is roughly 10 km, as suggested by the maximum depth of seismic faulting, the model yields spacings for the incipient Basin and Range structures of about 25–60 km, in excellent agreement with observation.

Bernard Hallet

Papers

Unstable extension of the lithosphere: A mechanical model for basin-and-range structure